The present invention has particular application to cellular mobile radio systems operating according to the GSM, DCS1800 or the PCS1900 standards. Systems operating according to these standards, including derivatives thereof, will be hereinafter be referred to as “GSM-type” systems. It should, however, be noted that the present invention is not restricted to a GSM-type system.
Cellular mobile radio systems, such as GSM-type systems, comprise a fixed part having switching elements and radio elements providing radio coverage in a plurality of cells, and mobile stations (MSs) for communicating with the fixed part of the system. Cells are often logically grouped into location areas. Within each cell, radio coverage is provided by one or more radio elements, which, in the case of GSM-type systems, comprise elementary transceivers termed TRXs (Transmitter Receiver). Radio elements are grouped within Base Transceiver Stations (BTS) which communicate with Base Station Controllers (BSC). The interface between a BTS and BSC, within a GSM-type system, is referred to as the A-bis interface, and individual links (generally comprising two physical channels) on this interface are termed A-bis links. BSCs communicate with mobile switching centres (MSC) via what is termed the A interface.
In a GSM-type system a specific radio frequency for each cell always plays a very important role in the operation of that cell. This frequency, which is used by control channels, is known as the Broadcast Control Channel (BCCH) frequency. It is characterised by continuous emission, fixed transmission power level and prohibition on frequency hopping. For this reason it is also sometimes called the cell “beacon frequency”. In a GSM-type system each cell is allocated a respective BCCH frequency and, since there are generally more cells in a network than different such frequencies available, the same BCCH frequency may be used by many cells in the network. Normally a particular BCCH frequency will not be used by multiple cells that are geographically close to each other, to minimise interference. Nonetheless, owing to the limited number of frequencies available it is possible for an MS to receive control channel signals on the same BCCH frequency from two or more different cells.
Another identity that is used in a GSM-type system is the Base Station Identity Code (BSIC), which is typically allocated to a group of cells adjacent to one another. The BSIC allows MSs to discriminate between different cells transmitting their control channels on the same BCCH frequency. The BSIC comprises a network “colour code” (NCC) and a base station colour code (BCC). The pair of BCCH frequency+BSIC is often used in GSM-type systems to identify a cell for radio purposes such as handover. BCCH frequency+BSIC is normally unique within a local geographic area, but not necessarily unique within a network.
MSs continuously make measurements of the receive level on the BCCH frequency of neighbour cells. This is to help choose the best serving cell when the MS is idle (“camping on”), and to aid the cell handover procedure when the MS is active—an active MS is one that is:    (a) performing a Location Update;    (b) engaged in set-up for a Mobile-Originated or Mobile-Terminated call;    (c) engaged in an active Mobile-Originated or Mobile-Terminated call;    (d) allocated a traffic channel or data channel on the air interface for any other reason—e.g. sending or receiving packet-switched data or a Short Message Service (SMS) message.
In order to speed and simplify the task of the MS in scanning for neighbouring cell BCCH frequencies, the network explicitly provides a list of BCCH frequencies that the MS should monitor. This list is known as the BCCH Allocation list or BA list. It is continuously sent out on the broadcast channel of each cell in order to be received by MSs that are in idle mode. It is also continuously sent to each MS that is in active mode on a Slow Associated Control Channel (or SACCH) associated with the active traffic or data channel. The two BA lists (the one transmitted to idle MSs and that transmitted to active ones) are known as the BA(BCCH) and BA(SACCH) respectively. Note that the BA(BCCH) and BA(SACCH) do not necessarily contain the same list of frequencies.
In a GSM-type system the BA(SACCH) downloaded to an active MS may be a cell default list that is identical for all active MSs in the cell, or it may be a channel-specific list associated with the current use of a particular air-interface channel by a particular MS. A channel-specific BA(SACCH) may be different from the cell default BA(SACCH). Whether a channel-specific BA(SACCH) or a cell default BA(SACCH) is in use by a particular active MS, the method of download of the BA(SACCH) over the air interface is the same—on the Slow Associated Control Channel as described above. However the way a channel-specific BA(SACCH) and cell default BA(SACCH) are set up in the BTS by the BSC is different.
The cell default BA(SACCH) is sent to the BTS by the BSC in an A-bis SACCH FILLING message on the A-bis interface between the BSC and BTS. The BTS stores this BA(SACCH) and autonomously transmits it on the downlink SACCH to each active MS that is not using a channel-specific BA(SACCH). In practice the BSC may not send SACCH FILLING messages to the BTS very often; for example it might be only on BTS or BSC software reboot, or when a frequency plan or neighbour cell list is changed. Thus days or weeks may elapse between transmission of the cell default BA(SACCH) over the A-bis interface from a BSC to a BTS.
As mentioned above the GSM specifications allow an active MS to have a different BA(SACCH) to the cell default BA(SACCH); that is, a channel-specific BA(SACCH). In this case the BSC is directly responsible for programming the channel-specific BA(SACCH), by sending it to the BTS either in an A-bis CHANNEL ACTIVATION message that activates an air-interface channel or in an A-bis SACCH INFORMATION MODIFY message relating to an already-active air-interface channel. Most current GSM-type systems tend to use a cell default BA(SACCH) rather than channel-specific BA(SACCH).
As also mentioned above, MSs make measurements of the receive level on the BCCH frequency of neighbouring cells. When in active mode (e.g. when a telephone call is taking place) these measurements are periodically reported to the network. This enables the network to make decisions about the need for “handing over” to a cell offering better signal quality than the current serving cell. A Direct Transfer Application Part (DTAP) Measurement Report message is used for this purpose—see technical specification 3GPP TS 44.018 “Mobile radio interface layer 3 specification; Radio Resource Control (RRC) protocol”, section 10.5.2.20. These measurements are sent at a rate of about two per second.
Constraints on the maximum size of signalling messages sent over the air interface limit the number of neighbour cells for which receive level measurements can be reported. In a GSM system this limit is six. In the event that the receive level on the BCCH frequency of more than six neighbour cells is measured, only the measurements corresponding to the six neighbour cells with best receive level are reported. The constraint on the size of signalling messages on the air interface also affects the way in which neighbour cell measurements are reported. Each neighbour cell measurement consists of three values:
BCCH-FREQ-NCELL i
BSIC-NCELL i
RXLEV-NCELL i
The BCCH-FREQ-NCELL i indicates the BCCH frequency of the ith neighbour cell, where i is between 1 and 6 for GSM networks. However B CCH-FREQ-NCELL i is not the absolute radio frequency of the neighbour cell beacon signal, but an index, starting with 0, of the position of the neighbour cell beacon frequency within the BA(SACCH) for the serving cell. Thus the BA(SACCH) needs to be known before the neighbour cell frequency, and hence neighbour cell identity, can be determined. BSIC-NCELL i and RXLEV-NCELL i are the BSIC value and received signal strength respectively of the ith neighbour cell. More details of the semantics and coding of these parameters are contained in 3GPP TS 44.018 cited above, Table 10.5.2.20.1 and Figure 10.5.2.10.1.
The neighbour cell measurement reports described above exist primarily for the network to manage handovers between cells. However the reports contain valuable data on the radio frequencies and signal levels currently being measured by the MS. Therefore the neighbour cell measurement data can be used for other applications in addition to the control by the network of handovers. Such other applications include cell RF planning (for example see U.S. Pat. No. 6,192,244), monitoring quality of service (QoS) of the radio interface, and determining MS position (for example see EP 1 304 897). These other applications for the measurement reports may be implemented by the network elements such as the BSC. Alternatively an external link monitoring system may also be used, as described in the two patent references cited above.
Any application that wishes to make use of neighbour cell measurement reports needs to know the BA(SACCH) downloaded from the serving TRX to the MS in order to be able to convert the BCCH-FREQ-NCELL i contained in the DTAP Measurement Report messages into the corresponding absolute radio frequency, and from that derive the neighbouring cell identity.
An issue for an application based on link monitoring is obtaining the BA(SACCH) currently being used by each MS. In particular, as explained above, the cell default BA(SACCH) may be transmitted only very infrequently over the links between the BSC and BTS, and hence it could be a long time (days or weeks) before use can be made of neighbour cell measurement reports. In addition there is a more subtle difficulty associated with BTS restarts. If a BTS is restarted it is likely that a cell default BA(SACCH) will be downloaded over the A-bis interface using an SACCH FILLING message. However a BSC or BTS restart can result in a re-mapping of the A-bis signalling channels within the timeslots on the physical links between the BSC and BTS. Those skilled in the art will know that it is possible to search for signalling channels carried in timeslots on such TDM links—for example by looking for valid high-level data link control (HDLC) frames within timeslots. Then methods such as those described in U.S. Pat. No. 6,088,587 may be used to re-discover the signalling channels to each TRX at the BTS. However, it is highly likely that the SACCH FILLING messages transmitted at BTS restart will be missed by the link monitoring system while signalling channel discovery takes place after the restart.
Existing solutions for BA(SACCH) configuration or discovery include:                1) Manually configure the BA(SACCH) for each BTS into the link monitoring system. This is both error prone and inconvenient.        2) A method of BA(SACCH) discovery, such as that described in U.S. Pat. No. 6,192,244, that does not rely on seeing SACCH FILLING messages. However this method is described in the context of an application analysing neighbour cell allocation. The method is not real-time in nature, requiring collection of quantities of A-bis signalling data and then analysing it (the example described refers to data collection over a period of days). Furthermore, the correlation algorithms employed by the method are complex.        
It would therefore be advantageous to have another method of discovering the cell default BA(SACCH) that allows the neighbour cell measurements in Measurement Report messages to be associated with the correct cells. This invention provides such an alternative method.